1. Field of the Invention
The invention relates generally to an interface for use in connecting a gas chromatograph or other gas introduction means to a mass spectrometer or other mass measurement means in order to allow for greater detection sensitivities in the mass measurement means. The invention relates specifically to a laminar flow, flow tube ion reactor cell in which specific ion species formed by radioactive bombardment of a gas mixture are reacted with a gaseous sample introduced into the tube, forming ionized species of the gaseous sample, which are then introduced into a collisional dissociation chamber, if needed, and then into a multistage pumped spectrometer, resulting in a substantial increase in the detection sensitivity for the mass spectrometer.
2. Prior Art
The coupling of a gas chromatograph to a mass spectrometer is known in the prior art. However, the sensitivity of the output of the mass spectrometer is limited both by the method of ionization and by the quality of input from the gas chromatograph (GC). The choice of GC operating conditions, sample separating column and sample separating stationary phase can greatly enhance or detract from the output to the mass spectrometer. The sensitivity of the coupled gas chromatograph/mass spectrometer system is limited by, among other things, the nonspecific ionization of sample species, background noise arising from effluent from the chromatograph column, the number and concentration of sample species contained in the sample gas, the loss of sample on the column material, the efficiency of the GC in separating interfering sample species, and the finite peak width of each sample species eluting from the gas chromatograph.
There have been attempts at developing methods or apparatuses for connecting a chromatograph to a mass spectrometer in such a manner so as to separate undesired chemicals from desired chemicals in the chromatograph prior to introducing the sample to the mass spectrometer. Reducing the complexity of the sample matrix introduced to the mass spectrometer reduces the complexity of the mass spectra created by the mass spectrometer. One such method and apparatus is described in the patent to Caroll et al, U.S. Pat. No. 3,639,757, which discloses an apparatus and method for the analysis of discrete samples. Caroll et al discloses the injection into and volatization of a sample within a first chamber. The sample is ionized with reacting ions produced in the first chamber and the reacted sample ions are directed by a drift field to a second chamber having a lower pressure than the first chamber. Within the second chamber, the ions are analyzed.
The disadvantages of the Caroll patent include a relatively short reaction time and relatively low pressure chambers resulting in lower sensitivity within the mass analysis device. Also due to the relatively short reaction time, the efficient use of the sample is relatively low and the selection by proton or electron affinity is diminished. Additionally, old sample gas remains in the first chamber, thus resulting in a memory effect within the first chamber. Further, uniform laminar flow is not achieved due to the presence of electrodes and guard rings within the first chamber. The ion chemistry in the Caroll device which finally leads to the product ion of interest also can be very complex, making the apparatus difficult to calibrate. The Caroll device also lacks a dissociation chamber to remove clusters which will cause unwanted peaks in the mass spectra.
A second apparatus and method for increasing the sensitivity for detecting low concentrations of sample gases is described in Ketkar, S. N. et al., Atmospheric Pressure Ionization Tandem Mass Spectrometric System For Real-Time Detection Of Low-Level Pollutants In Air, 61 Analytical Chemistry 260-264 (1989). The Ketkar article describes an ionization system for detecting very low levels of contamination in air. The specific system described uses a point-to-plane corona discharge with means to produce primary ions which ionize the trace molecules in a sample gas. A low pressure declustering region helps remove water cluster ions and a tandem mass spectrometric system is used to detect trace molecules.
There are several disadvantages inherent in the Ketkar system, including several of the disadvantages listed above. Relatively short reaction times and relatively low pressures decrease the sensitivity of the mass analysis device. The relatively high temperature ion reaction region may cause fragmentation or radical formation of unwanted species within the reaction region. Further, the Ketkar apparatus does not act as a wall-less reaction region, thus resulting in the possibility of a memory effect. Again, ion chemistry is complex and variable, and thus the instrument is difficult to calibrate. The low pressure cluster removal means has comparatively low efficiency for removing these clusters.
The patent to the University of Toronto, U.K. Patent No. 1582869, discloses a gas curtain device and method for transferring matter between a gas and a vacuum. A second patent to the University of Toronto, U.K. Patent No. 1584459, based on the above patent, discloses a method of focussing and dissociating trace ions. The first patent includes a flow tube having a centrally located axial electrode to induce ion drift into a gas curtain to facilitate the transfer of the sample ions and not buffer gas into a mass analyzer. The present invention has been used in conjunction with devices similar to this one; however the central electrode, on which sample gas species could be absorbed and later desorbed, was found to be unnecessary, and for relatively clean sample gas the curtain gas also is not needed. When the present invention was used in conjunction with a dissociation device, a separate chamber for dissociating clustered ions was found advantageous.
Each of these components has a distinct disadvantage when compared to the present invention. First, the centrally located electrode may disrupt the axial flow of the sample gas and prevent uniform laminar flow through the reaction region. Second, in contrast to the relatively high pressure laminar flow in the present invention, a drift field is required. In the present invention interface, sample gas ions will naturally remain near the axis constrained by diffusion in the relatively high pressure flow tube until they reach the relatively low pressure collision chamber or analyzer. Third, the prior art requires a gas curtain. In contrast, the clean buffer gas and preseparated sample gas in the present invention, uninterrupted by the presence of any axial electrode or other surfaces, obviates the need for a gas curtain, as the clean, dry buffer/sample gas presents no problems upon entering the collision chamber or analyzer region. Fourth, the prior art are of use downstream from where the selected ion chemical-ionization process takes place; that is, downstream from where the present invention is located. This prior art is useful primarily with the present invention as shown in FIGS. 3 and 4 herein.
Atmospheric pressure ionization mass spectrometry (APIMS) has proved to be an extremely sensitive method for detecting gas phase species at ultratrace levels. Currently, most of these methods are employed only for analysis of bulk phase samples. Therefore, their extreme sensitivity is rather limited to only those species having relatively high proton or electron affinities. The typical hierarchy of potentially stable product ion species present in gas samples limits the present applicability of chemical ionization mass spectrometry methods to relatively few, very stable species. Sensitive detection of species having relatively low proton or electron affinities can be achieved by coupling atmospheric pressure chemical ionization/MS with a technique such as gas chromatography (GC) which separates the component(s) of interest in the sample matrix from interfering high affinity species. Thus, a potentially large number of species may be detected with extreme sensitivity using combined GC/APCI/MS. However, to this date, the powerful capabilities of this technique have hardly been recognized.
The range of species measured by chemical ionization techniques also has been limited by the relatively crude manner in which these techniques have been previously applied. Most chemical ionization techniques directly ionize the sample gas being studied using either radioactive alpha or beta sources or a corona discharge. This poses several problems and drawbacks:
1. Both alpha and beta irradiation of a sample gas impart only half of the energy into the production of ions. The ionization efficiency is even lower for corona discharge.
2. Both alpha and beta sources produce metastable and neutral radical species at a rate at least as high and probably higher than the initial ion production rate. Unless the ions are extracted by strong electric fields, most ions are lost by ion-ion recombination, potentially forming additional radical species.
3. Even if ions are extracted from the ion source region, the initially formed neutral radicals are not. They are only removed by the gas flow through the ion source region and have a significant time to react with the trace species in the sample gas. Radical production rate for a 10 mc .sup.63 Ni beta source of 0.1 - mc .sup.241 Am source can reach 10.sup.9 -10.sup.10 /sec which at gas flows of 10 cm/sec through the source region can result in a concentration of 10.sup.8 -10.sup.9 radicals or chemically altered species. In most applications this causes major interferences preventing a sensitive detection of the sample species in the low parts-per-trillion (pptrv) or sub-pptrv range.
4. Corona sources can cause at least as much alteration of the sample chemistry as direct radioactive sample irradiation.
Compared to the prior art, the present invention allows for the separation of the ionization and reaction regions. The present invention allows for the preparation of a single selectable initial reactant ion species reducing or in many cases removing the need for constant calibration. The present allows for longer reaction times and higher pressure chambers thus resulting in higher sensitivity for the mass analysis means, a more efficient use of sample, and the creation of an essentially wall-less laminar flow reaction region, greatly reducing or eliminating the reactions on the tube walls and possible memory effects. A low temperature ion reaction region results in essentially no fragmentation or radical formation within the reaction region. Laminar flow is achieved in the present invention through the use of turbulence-reducing screens and the elimination of guard rings or electrodes. The specific collisional dissociation chamber developed for the present invention results in a higher removal rate for weakly bound clusters such as water clusters while minimizing the dissociation of core ion species.
The present apparatus does not suffer from the difficulties encountered with previous chemical ionization techniques because it does not directly ionize the gas being sampled. Instead, the present apparatus forms ion species in a buffer gas and allows sufficient time for most metastable or radical species to be removed before they are allowed to interact with the gas sample being analyzed. The present technique also differs from conventional chemical ionization methodology in that it uses a single specific core ion species to react with the trace compound to be measured. This is accomplished by forming the initial reactant ions in a tailor made buffer gas. Thus, the terminal ion to be detected is formed in a single known reaction (often a fast proton or electron exchange reaction) with the compound to be detected. Therefore, the detection sensitivity of the measurement is known or can be measured, and is dependent on one reaction rate constant and the reaction time. It is not dependent on the other unknown constituents of the gas being sampled. Thus, the system need not be calibrated each time new sample gas is added. If the species to be detected forms a sufficiently stable ion such that once formed it will not react further, then the present device often can be operated in conjunction with a mass spectrometer on a continuous basis (no GC). This is possible as long as the initial reactant can be maintained as the predominant ion species present by: (1) choosing a sufficiently stable reactant ion; (2) reduction of reaction time; (3) sample dilution; or (4) some combination of the above possibilities.
The present invention makes possible the direct detection and quantification of gaseous samples in air at parts-per-trillion (pptrv) and sub-pptrv levels involving no preconcentration. Generally, previous methods used to analyze gaseous samples using gas chromatography/mass spectrometry (GC/MS) involve detection systems with much lower sensitivities (typical commercial MS detectors: typically in the ppb range and above) than the present invention and, therefore, require preconcentration of the gaseous sample for levels in the lower pptv range. Preconcentration (for instance adsorption on solid adsorbent, or cryogenic trapping) bears several serious disadvantages such as, for example, the potential occurrence of artifact reactions of the sample gas with other reactive species during preconcentration or injection of the preconcentrated sample and taking time. In contrast, the present invention is essentially free of interferences, is highly sensitive and highly selective, and involves direct identification of the species of interest by single or tandem mass spectrometry.